1. Field of the Invention
The present invention relates to improved linearity of an active circuit, and more particularly, to an active circuit having improved linearity using a main circuit unit and an assistant circuit unit.
2. Background of the Related Art
Recently, the reference for the linearity of semiconductor devices is IP3 (3rd Intercept Point). IP3 means that two points meet under the assumption that the fundamental output and the IM3 (3rd Inter-modulation) component continue to increase without being saturated.
At this time, IP3 is classified into IIP3 (Input IP3) and OIP3 (Output IP3). IIP3 is used when IP3 is read from the viewpoint of the input side, and OIP3 is used when IP3 is read from the viewpoint of the output side.
Characteristics regarding the conventional active element will be schematically described with reference to the drawings, and problems thereof will be also described.
FIG. 1 is a graph showing the secondary differential coefficient of the transconductance of a conventional common gate circuit.
The conventional common gate circuit is usually used as a circuit for generating 50 (ohm) being RF input impedance since input resistance can be designed to have 1/gm.
In FIG. 1, a region A and a region B divided and shown.
At this time, the coefficient related to the transistor linearity is gm″ being the secondary differential coefficient of the transconductor. In the region A being a main use region, the value of gm″ has a positive value. The linearity of the circuit becomes low due to the positive value.
FIG. 2 is a graph showing the secondary differential coefficient of the transconductance of a conventional common drain circuit.
The conventional common drain circuit is usually used as a buffer circuit since it has high input impedance and low output resistance.
In FIG. 2, a region A and a region B are divided and shown.
At this time, the coefficient related to the transistor linearity is gm″ being the secondary differential coefficient of the transconductor. In the region A being a main use region, the value of gm″ has a negative value. The linearity of the circuit becomes low due to the negative value.
Further, the linearity of the conventional common drain circuit can be expressed into the following Equation (1).
[Equation 1]IIP3SF=IIP3NMOS·(1+T)3/2 Where T=gm·(RL//RO)  (1)
As can be seen from Equation 1, if the load impedance is higher, the linearity increases since feedback increases. In the case of a high frequency circuit, however, it is difficult to obtain the linearity since the load impedance is difficult to be higher.